Normally, a mobile station (MS) in active mode in a cellular wireless communication system is handed over from one cell to the next as it moves through the system, and data can be transmitted and received without significant interruptions due to these handovers.
A handover (HO) procedure can consist of many steps. In many cellular wireless communication systems a HO is: 1) network controlled, i.e. the MS is commanded by the network when to connect to another cell; 2) prepared, i.e. the target cell to which the MS is moving to is prepared; and 3) MS assisted, i.e. the MS provides measurement reports before HO to the serving cell to assist the decision to do HO preparation of target cell(s), and when to leave the serving cell/connect to the target cell.
In the context of HO, the serving cell before HO is often referred to as the source cell. After successful HO the target cell becomes the new serving cell. In Long Term Evolution (LTE) the HO is a “hard handover”, which means that the UE radio link is switched from one (source) cell to another (target) cell. In Universal Mobile Telecommunications System (UMTS) hard handovers are used exclusively for Time Division Duplex (TDD) mode, but may also be used for Frequency Division Duplex (FDD) mode.
In the following discourse, the focus is on the intra freq LTE HO procedure, but the procedures are similar for the LTE inter Radio Access Technology (RAT) and LTE inter frequency HO procedures. The intra E-UTRAN in RRC_CONNECTED state is a User Equipment (UE) assisted network controlled HO, with HO preparation signalling in E-UTRAN.
A HO is initially triggered by a measurement report sent from a UE to a serving eNB (E-UTRAN NodeB). The serving eNB configures how the UE shall take measurements, and under what conditions a measurement report shall be triggered and sent to the eNB.
To assist mobility control decisions, the UE can measure several different cells and report the results to the network. Different networks and network deployments can have different detailed behaviour, but in most systems it is natural to trigger HO when signal reception from a target cell is better than from a source cell.
For the case of intra-frequency HO in a reuse-one system (i.e. in a system where the source cell and the target cell uses exactly the same frequency resources) there are strong interference management benefits in (always) keeping the UE connected to the cell with the best signal strength. In the measurement report, the UE includes the reason for the trigger of a HO, e.g. target cell signal stronger than serving cell signal, and measurements of a Reference Signal Received Power (RSRP) or Reference Signal Received Quality (RSRQ) of the serving cell and several neighbour cells including the target cell. To reduce ping-pong effects where a UE is handed over repeatedly between two cells a HO offset is often added to the HO trigger condition: target cell signal should be better than the serving cell signal by an offset, wherein the offset value >0 dB.
When a serving eNB receives a measurement report from a UE and if the eNB wishes to HO the UE to another cell, the eNB performs a HO preparation to that cell. HO preparation involves a signalling exchange between one (serving) eNB and another (target) eNB. The source cell requests the HO (Handover Request) and passes over UE context information; and the target cell decides if it can admit the UE (Call Admission Control) and either accepts or rejects the HO. In an acceptance message (Handover Request Ack), the target cell includes parameters required by the UE to allow it to communicate to the target cell—these parameters being grouped into a transparent container. The source cell may prepare multiple cells for HO.
Following a successful preparation, the HO execution takes place. The source cell issues a HO Command to the UE—this is the RRCConnectionReconfiguration message and carries the transparent container. If, and when, the UE receives this message correctly the UE synchronises to the new target cell and sends a synchronisation message on the Random Access Channel (RACH). The target cell then issues an allocation to the UE so that the UE can send a HO confirmation message to the target cell (RRCConnectionReconfiguration-Complete message).
In the final steps (Handover Completion), which do not involve the UE, the source eNB (serving the source cell) is able to forward data (un-acknowledged downlink packets) to the target eNB (serving the source cell), and the S1-U interface from the Serving Gateway (S-GW) must be switched from the source to the target cell (“path switch”). Finally, if the handover is successful, the target eNB issues a UE Context Release message to the source eNB.
However, it is possible for a HO to fail at different points because of a Radio Link Failure (RLF) or a failure by the RACH. A RACH failure during a HO is called “Handover Failure” in 3GPP TS36.331, but for the remainder of this disclosure the term HO failure is used to comprise both RLF and RACH failures.
After a HO failure, the UE attempts a RRC re-establishment which is described in specifications 3GPP TS36.300 and 3GPP TS36.331. The UE firstly tries to find the strongest cell that it can detect (“cell selection”), and then the UE sends a RRCConnection-ReestablishmentRequest to the cell that it has selected. If this selected cell has prior knowledge of the UE and details regarding the UE connection (e.g. security parameters, this is called the “UE Context”) then the cell can send a RRCConnectionReestablishment and the re-establishment will succeed which means that the UE remains in Radio Resource Control (RRC) connected state.
If however the UE context is lacking, the re-establishment request is rejected and the UE drops to RRC idle state, which results in further delay before the UE can transit to RRC connected state and recommence any data communication. The “UE Context” may be passed to a cell during the HO procedure or at some other point in time. This transfer is called HO preparation. Note also that the RRCConnectionReestablishmentRequest carries three fields, the Cell Radio Network Temporary Identifier (C-RNTI) of the UE in the serving cell where failure occurred, the Physical Cell Identity (PCI) of this cell, and the shortMAC-I calculated using the Identity (ID) of the re-establishment cell.
The hard HO in the Universal Mobile Telecommunications System (UMTS) is very similar in many respects to the above description—i.e. also being UE assisted but network controlled, which means that the UE is configured to send triggered measurement reports but the network decides when to execute the HO; exploits preparation (using Radio Link Setup procedure); is “backward” HO, which means that the source cell sends the HO command to the UE and the UE replies to the target cell; and is completed by inter-node signalling.
Furthermore, RLF is described in specifications 3G PP TS36.300 and 3GPP TS36.331. One form of RLF is driven by out-of-sync detection by Layer 1. A radio problem detection procedure is started when a UE receives a certain number of consecutive “out-of-sync” indications from lower layers. The number of consecutive indications is specified by the threshold N310. When this happens, the UE starts a timer T310. In case the UE receives a certain (N311) consecutive “in-sync” indications from lower layers while T310 is running, the UE shall stop the timer and return to normal operation.
Following the declaration of a RLF, the UE attempts cell selection. If the UE manages to find a cell to connect to within the cell selection phase, the UE will attempt to re-establish RRC to this cell. If, on the other hand, the UE does not find a cell within the cell selection phase (T311), the UE goes back to idle mode and may start looking for cells on other RATs, examples of which are LTE, UMTS, WiMaX and GSM EDGE Radio Access Network (GERAN).
RLF can also be declared by the Radio Link Control (RLC) layer of the UE when a maximum number of transmissions have been reached for transmission of an uplink RRC signalling packet, but the packet has still not been delivered successfully. Additionally, if the random access during the HO fails (T304 timeout) the UE behaves as if a RLF had occurred. In the present discourse the term RLF relates to any of the above mentioned events.
Moreover, a RLF report was introduced in 3GPP Rel-9 to enable a eNB receiving a RLF report to distinguish between Mobility Robustness Optimisation (MRO) related problems and coverage problems. This was done by including a set of neighbour cell measurements indicating the signal strength at the time of failure. With the help of this, the eNB is able to see if there is an alternative neighbour cell that might have been used, or if there is no neighbour detected in the case of a coverage hole.
The RLF report carries information about:
Serving cell RSRP, and optionally RSRQ;
Neighbours cell RSRP/RSRQ; and
May also indicate the strength of detected inter-RAT neighbour cells.
In Rel-9, if a RLF during a HO is followed by a successful RRC Re-establishment, it is possible to include a RLF Report in a RLF INDICATION message that is sent from a eNB where re-establishment takes place to a eNB that was serving the UE at the point of RLF. The capability of the UE to provide the RLF Report is indicated by a flag in the RCConnectionReestablishmentComplete message. The RLF Report is then provided to the eNB where re-establishment took place using the UE Information procedure.
One remaining problem in Rel-9 is that the UE is only able to send a RLF report if RRC re-establishment is successful. And this is only possible if the cell receiving the RLF report has the context of this UE (it is “prepared” for the HO). In most HO failure cases, the HO is executed too late and the cell where the UE attempts RRC re-establishment is not prepared, so the UE can not send the RLF report.
In order to mitigate this, there have been suggestions to allow the RLF reporting to take place also after the UE has gone back to idle mode. This would mean that the UE reports when attempting RRC establishment.
Furthermore, in 3GPP there has been considerable study into Self-Organising Networks (SON) for LTE. One part of this is the Handover Parameter Optimisation also known as the above mentioned MRO which is aiming at optimising mobility parameters. It has not been specified which 1-10 parameters shall be optimised, but examples include the HO hysteresis (also called offset) and the Time to Trigger (TTT) parameters. The aims of the optimisation are to reduce HO failures whilst at the same time not having more HOs than are necessary. The MRO functionality is distributed in the Evolved-UTRAN (E-UTRAN), i.e. every eNB has its own MRO optimisation function. To assist optimisation, signalling has also been defined between eNBs to help identify HO failure events.
The following is the text describing the use-case of Handover Parameter Optimisation also known as MRO in section 22.5 of specification 3GPP TS36.300, 9.2.0:                One of the functions of Mobility Robustness Optimization [MRO] is to detect RLFs that occur due to Too Early or Too Late Handovers, or Handover to Wrong Cell. This detection mechanism is carried out through the following procedures:                    [Too Late HO] If the UE attempts to re-establish the radio link at eNB B after a RLF at eNB A then eNB B may report this RLF event to eNB A by means of the RLF Indication Procedure.            [Too Early HO] eNB B may send a HANDOVER REPORT message indicating a Too Early HO event to eNB A when eNB B receives an RLF Indication from eNB A and if eNB B has sent the UE Context Release message to eNB A related to the completion of an incoming HO for the same UE within the last Tstore_UE_cntxt seconds.            [HO to Wrong Cell] eNB B may send a HANDOVER REPORT message indicating a HO To Wrong Cell event to eNB A when eNB B receives an RLF Indication from eNB C, and if eNB B has sent the UE Context Release message to eNB A related to the completion of an incoming HO for the same UE within the last Tstore_UE_cntxt seconds. The indication may also be sent if eNB B and eNB C are the same and the RLF report is internal to this eNB.                        The detection of the above events is enabled by the RLF Indication and Handover Report procedures.        The RLF Indication procedure may be initiated after a UE attempts to re-establish the radio link at eNB B after a RLF at eNB A. The RLF INDICATION message sent from eNB B to eNB A shall contain the following information elements:                    Failure Cell ID: PCI of the cell in which the RLF occurred;            Reestablishment Cell ID: ECGI of the cell where RL re-establishment attempt is made;            C-RNTI: C-RNTI of the UE in the cell where RLF occurred.            shortMAC-I (optionally): the 16 least significant bits of the MAC-I calculated using the security configuration of the source cell and the re-establishment cell identity.                        eNB B may initiate RLF Indication towards multiple eNBs if they control cells which use the PCI signalled by the UE during the re-establishment procedure. The eNB A selects the UE context that matches the received Failure cell PCI and C-RNTI, and, if available, uses the shortMAC-I to confirm this identification, by calculating the shortMAC-I and comparing it to the received IE.        The Handover Report procedure is used in the case of recently completed handovers, when an RLF occurs in the target cell (in eNB B) shortly after it sent the UE Context Release message to the source eNB A. The HANDOVER REPORT message contains the following information:                    Type of detected handover problem (Too Early HO, HO to Wrong Cell)            ECGI of source and target cells in the handover            ECGI of the re-establishment cell (in the case of HO to Wrong Cell)            Handover cause (signalled by the source during handover preparation)                        
Inter RAT Mobility
HO between different RATs, i.e. inter RAT HO, can have different causes. Two examples could be:                Coverage—the coverage on a current RAT is not sufficient, but there exist coverage on other RATs. Therefore, the UE may be ordered to HO to another RAT;        Capacity—the capacity in the current RAT may not be sufficient, but there exist available capacity in another RAT covering the same area.        
For inter RAT mobility based on coverage there is typically two absolute signal strength, or signal quality thresholds: criteria in source RAT and target RAT. The target RAT criterion can be used to set the threshold for which a UE in a LTE system is expected to survive in the target RAT. The source criterion can be used to adjust at what time the UE starts performing inter RAT measurements.
Using a too low criterion for the serving cell leads to excessive measurements and using a too high criterion may lead to dropped calls since the UE is not able to find an alternative before the quality to the serving cell is too poor to use for further communication.
Typical events available for measurement configuration according to specification 3GPP TS36.331 is:                Event A1: Serving becomes better than threshold.        Event A2: Serving becomes worse than threshold.        Event A3: Neighbour becomes offset better than serving.        Event A4: Neighbour becomes better than threshold.        Event A5: Serving becomes worse than threshold1 and neighbour becomes better than threshold2.        Event B1: inter RAT neighbour becomes better than threshold.        Event B2: Serving becomes worse than threshold1 and inter RAT neighbour becomes better than threshold2.        
Here “serving” refers to a serving or source cell, and “neighbour” refers to a detected neighbour cell. The different events compare signal strength (or quality) of cells against fixed thresholds, or the values of serving and neighbour cells are compared (including a hysteresis value as described above).
One typical implementation of an inter RAT HO algorithm (coverage HO) would be as follows:                Inter RAT measurements are started when the quality of the serving cell is below an acceptable level, using event A2.        At that time measurements gaps are configured (if needed) and the UE is configured to report cells from another RAT using event B1 or B2        
Inter RAT MRO
An inter RAT MRO functionality in a eNB could adjust the following parameters:                When measurement on the other RAT cells starts (A2) and when report should be triggered (B2_threshold1));        Requirements on the target cell before reporting (B1 and B2_threshold2);        Setting of different offsets for different frequencies (the offset is set via parameter ofn).        
The error cases for inter RAT MRO is slightly different compared to MRO for HOs within LTE (intra-frequency or inter-frequency handover). Different error cases could be, where IRAT denotes inter RAT:                IRAT too late—the threshold requirements for serving cell is set too low, causing the UE to move out of the serving cell before the measurement reports can be started, or the HO can be executed;        IRAT too early—the threshold requirements on the target cell are too low, causing the HO to fail, or an RLF occur shortly after HO;        IRAT wrong RAT—RLF occurring shortly after an HO to another RAT leading to a re-establishment in a third RAT;        IRAT frequent—the requirements on the serving cell is set too strict, casing inter RAT HO even if an intra LTE HO would have been possible;        IRAT ping pong—the requirement on the target cell is too low compared to the requirement on the source cell when the UE has changed RAT;        IRAT rapid HO—Rapid HO to another RAT occurring after HO.        
It may be assumed that the “IRAT too late” is the most important failure case, at least initially, since the LTE system may have spotty coverage and will suffer if a HO to legacy systems is not performed in time. The occurrence of “IRAT too late” can be measured by counting the number of times a UE encounter a RLF in the cell, goes back to idle mode and manages to find a new cell in another RAT.
There has been a proposal to extend the existing intra LTE MRO to enable network nodes from different RATs to exchange information (e.g. NGMN Alliance, Handover Optimisation). In the “IRAT too late” example, this would mean that the cell where the UE manages to connect after the RLF will receive a report from the UE with details on the serving cell prior to the failure. This eNB would then need to send a message similar to “RLF indication” to the eNB handling the cell where the UE was connected to before the RLF. It has been discussed that the RAN Information Management (RIM) interface should be used to enable this information transfer between different RATs.
In order to collect and exchange information across RATs, the different RATs have to be able to decode information received from the UE, concerning another RAT and also send this information in a commonly agreed format between the RATs.
Further, the network interface most probable to be used is the RIM. This has been agreed to be used for other purposes for communication between different RATs, with the requirement that the signalling should be limited. The reason for this limitation is the concern about processing complexity for core network nodes. Hence it could be problematic also using this interface for inter RAT MRO.
There is therefore a need for a method for providing information in a cellular wireless communication system mitigating or solving the problems of prior art.